STAR Flow, Chirality and Vorticity PWG

[Sooraj Radhakrishnan <skradhakrishnan AT lbl.gov>]

Date: 02/07/2025

Participants: Fuqiang Wang, Haojie Xu, Jie Zhao, Jinhui Chen, Zaining Wang, Nu Xu, Prithwish Tribedy, Richard Seto, Subhash Singha, Isaac Mooney, Nihar Sahoo, Qian Yang, Guannan Xie, Yue-Hang Liang, Barbara Trzeciak, ShinIchi Esumi, Sooraj Radhakrishnan

Title: The non-linear response coefficient $\chi_{4,22}$ in Au+Au and U+U collisions

PAs: Jinhui Chen, Yugang Ma, Fuqiang Wang, Zainin Wang, Haojie Xu, Jie Zhao

Target journal: PRL 

Proposal page: https://www.star.bnl.gov/protected/lfsupc/jiezhao/UUchi422/v1/UUx422proposal.html

Presentation:  https://drupal.star.bnl.gov/STAR/system/files/PWGC_1.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that the analysis is mature and results are important and interesting, and the paper should move forward. The journal choice was found to be appropriate. The following points were discussed.

Q: In the conclusion you say the beta_4 value estimated from this analysis is consistent with nuclear structure theory calculations. What is the value from nuclear structure theory calculations? What is the uncertainty from this analysis?

A: On S.4 lower part, the value used by nuclear structure theory group in evaluating eccentricity is shown. This is not a calculation, but a value used by them in calculations. 
Q: The beta_4 values from AMPT and Hydro are a factor or 2 different, so the model uncertainty in this estimation is large
A: Yes
Q: So we cannot make the conclusion that the value from this analysis is consistent with nuclear structure theory calculations 
A: Yes, agree. We will remove that conclusion

Q: On Fig.4, can we label the y-axis with the quantity?

A: Yes 

Q: In Fig.4, in the calculations shown, is it assumed that the beta_4 in Au+Au is zero? 
A: Yes, it is assumed that beta_4 for Au+Au is zero
Q: Is this a valid assumption? Do we have evidence for this? 

A: This is an assumption we make

Q: How does the AMPT calculations with beta_4 = 0 look?
A: It would be similar to the Hydro calculation with beta_4 = 0

Q: In the title, what is non-linear response?
A: This is the coefficient Chi_4,22 which captures the non-linear response to the geometry

Q: Can we use beta_4 in the title? That would be more general than using technical term like non-linear response
A: Yes, agree. We will try to make the change 

Q: Why the deviation is seen only in most central collisions and not other centralities?
A: In central collisions there is more sensitivity to deformation

Q: We should try to quantify the uncertainty in the estimation of beta_4, from the assumptions and the models used 
A: Yes, we will try to do that

Q:  Have you tried to use the ratios of other quantities shown in Fig.3 to extract beta_4? Is Chi_4,22 most sensitive?

A: Yes, from model studies we see Chi_4,22 is most sensitive
Q: From Fig.3 ac_2 and EP correlation also shows similar deviation from 1
A: Yes, we havent looked at this in models 

Q: Have you tried if the v4 can be explained with the linear and non-linear components, as shown in ATLAS measurements?

A: We havent tried it
C: Could try it as an internal check
A: Yes

Q: If you change beta_2, do you see deviation from 1 for Chi_4,22 ratio? Why does the hydro calculation with beta_4 = 0 show deviation from 1?
A: We have checked in models that changing beta_2 within known limits doesnt cause deviation from 1 for the ratio. This can be seen in S.5. It is not clear why the hydro calculation with beta_4 = 0 deviates from 1

Q: Have you done estimate of non-flow in the results?
A: Yes, we have. This is in the analysis note. The non-flow contribution is small, as Chi_4,22 is evaluated as a ratio of ac_2{3} and v_2,4 

Q: In Fig.4, what would be the baseline? Is it at 1 or the hydro values?
A: It would be hydro
Q: Can we have AMPT calculation with beta_4 = 0
A: Yes, we will include that

Q: What is the beta_2 value in the two calculations in Fig.4? Are they the same? 

A: The ratio is insensitive to beta_2 

Q: How does the ratio change if beta_4 in Au+Au is changed from zero?
A: This is an assumption. We can vary beta_4 in Au+Au in some range and quote a systematics 

Q: In the equation on S.7, are there more terms in the expansion of v4?
A: No, these are the only two terms 

--

Sooraj Radhakrishnan

Research Scientist,

Department of Physics

Kent State University

Kent, OH 44242

Physicist Postdoctoral Affiliate

Nuclear Science Division
Lawrence Berkeley National Lab
MS70R0319, One Cyclotron Road

Berkeley, CA 94720

Ph: 510-495-2473

Email: skradhakrishnan AT lbl.gov